In recent years, improved fuel economy of automobiles and reduced weight of chasses have been increasingly demanded. To reduce the weight, the need for high strength steel sheet excellent in tensile strength and yield strength has been rising. However, along with the rise in strength, this high strength steel sheet has become difficult to shape. In particular, steel materials have fallen in elongation. As opposed to this, recently, TRIP steel (high residual austenite steel) high in both strength and elongation has come to be used for the frame members of automobiles.
However, conventional TRIP steel contains over 1% of Si, so there are the problems that the plating is difficult to uniformly adhere and the hot dip galvanization ability is poor. For this reason, hot dip galvanized high strength steel sheet reduced in the amount of Si and containing Al as an alternative has been proposed in Japan Patent No. 2962038 and Japanese Patent Publication (A) No. 2003-105486. However, the former continues to have a problem in the plating adhesion since the Si content is a relatively high amount of 0.53% or more. Further, the latter reduces the Si content to less than 0.2% to improve the plating adhesion, but residual austenite is built into it due to the relatively high cooling rate, so there was the problem that the cooling rate could not be stably controlled and therefore the quality of the material became unstable.
Further, there are also quite a few members which are worked by burring to expand the worked hole part and form a flange. Steel sheet also having a hole expandability as an important characteristic is therefore being demanded. Conventional TRIP steel meeting this demand changes from residual austenite to martensite after induced plastic transformation and has a large hardness difference from ferrite, so there is the problem of an inferior hole expandability. Further, due to the demands from auto manufacturers and home electrical appliance manufacturers for making steel sheet rust-proof, hot dip galvanized steel sheet has been spreading. In this way, various manufacturers have been changing over from conventional cold rolled steel sheet to surface treated steel sheet. Production setups have become necessary which shorten the production processes to enable large emergency and short delivery orders for surface treated steel sheet, in particular hot dip galvanized steel sheet, to be handled. However, in the case of high temperature annealed material or high strength steel sheet for producing the above-mentioned hot dip galvanization steel sheet, productivity is low due to the high temperature annealing. Therefore, in the case of large emergency orders/production of hot dip galvanized high strength steel sheet, there is the problem that production concentrates at the hot dip galvanization line having an annealing furnace before it and cannot be handled.
On the other hand, an ordinary continuous annealing line for annealing the sheet material of cold rolled steel sheet or electrogalvanized steel sheet generally has a high speed and high productivity. Regardless of this, there are problems that a change in production reduces the production load as explained above, sometimes there is no material being run, and the production line is idled. There was therefore the serious problem of an excess in production capacity.